1. Field of the Invention
The present invention relates to an apparatus for sensing NOx in a gas to be measured, having a sensor element made of an oxide, the resistance of the oxide varying in response to changes in NOx concentration of the gas if the oxide is contacted with the gas including NOx, and a measuring portion for measuring a resistance variation of the sensor element and for determining an NOx concentration in the gas to be measured. The present invention especially relates to an apparatus used preferably for sensing low concentration NOx in the atmosphere as respective NO concentration and NO.sub.2 concentration.
2. Related Art Statement
As a method of measuring an NOx concentration in a gas such as a fired gas from an incinerator, which includes an NOx component such as nitrogen oxide, it is known to sample the gas including an NOx component, in for example, a dust chimney, and to measure an NOx concentration of the sampled gas by means of an optical measuring apparatus. However, the optical measuring apparatus is expensive and the measurement time is long since the sampling operation is necessary.
In order to eliminate the drawbacks mentioned above, it has been proposed to use a direct insertion type semiconductor sensor. For example, in Japanese Patent Laid-Open Publication No. 6-222028 (JP-A-6-222028), an NOx sensor comprising a response portion made of an oxide having a predetermined perovskite structure, and a conductivity measuring portion for measuring a conductivity of the response portion is disclosed.
However, in the direct insertion type semiconductor sensor mentioned above, there is no countermeasure for an influence of O.sub.2 and CO components included in the gas to be measured with respect to the measured NOx concentration. Moreover, in the response portion, the resistance thereof is varied in response to the concentration of NOx(NO.sub.2 +NO). However, if a ratio of partial pressure between NO.sub.2 and NO, is varied, a resistance measured by the response portion is varied even for the same NOx amount. In this case, it is reasonable to conclude that the NOx component is not selectively measured. Therefore, in the direct insertion type semiconductor sensor mentioned above, there is a drawback in that the NOx concentration in the gas to be measured cannot be selectively measured in a highly precise manner, even though the semiconductor sensor is cheap and shows excellent response time as compared with the optical measuring apparatus.
Further, in order to solve the drawbacks mentioned above, the applicant discloses in U.S. Pat. No. 5,705,129 an NOx sensor comprising; an oxide sensor element; a catalyst arranged upstream of the oxide sensor element to maintain partial pressures of NO and NO.sub.2 in the measurement gas at an equilibrium state; a heater for controlling a temperature of the oxide sensor element; and an O.sub.2 sensor for a correction. However, also in the NOx sensor mentioned above, a target measurement gas is a fired exhaust gas from an incinerator as mentioned above. Therefore, the NOx sensor mentioned above cannot be used for measuring low concentration NOx in the atmosphere which is a target measurement gas of the invention.